A highly specific isozyme of cyclic nucleotide phosphodiesterase, PDE-f, is only present in the testis and appears concurrent with sexual maturation. The testis of the rat contains three isozymes of PDE: PDE- c, PDE-e, and PDE-f. PDE-c is the high Km cyclic AMP phosphodiesterase (6.5 x 10 minus to the 5th power M), while PDE-f is the low Km isozyme (2.5 x 10 minus to the 6th power M). PDE-f is localized to the nuclear fragment of the seminiferous tubule and is not present in spermatozoa. PDE-f disappears 18-19 days following hypophysectomy and is induced in the hypophysectomized animal by the combination of ICSH and FSH or testosterone and FSH. Current work centers around separation of the isozymes by column chromatography and determinations of kinetic parameters.